Multitubular membrane modules wherein a plurality of membrane tubes, e.g. 3 to 20, are disposed in a module housing are used to filter feed streams where a high membrane packing density is desirable. In order to keep the volumetric flow rate low, the membrane modules are connected in series wherein the concentrate stream from the first module is the feed stream for the next succeeding membrane module and so forth. The number of membrane modules connected in series may vary; however, the number is limited by the pressure drop through the membrane modules and their connecting fittings. Ultrafiltration membrane module systems employing a number of modules in series are used for the separation of solid-containing feed streams, such as, but not limited to: aqueous electrocoat paint streams; oil in water emulsions; aqueous polyvinyl alcohol; industrial waste streams; dairy streams; and fruit juice streams.
Generally, a tubular membrane system has an even number of membrane modules to reduce piping with the tubes connected in series with U-bend fittings at each end, except the initial feed inlet and final concentrate outlet. The feed inlet and concentrate outlet are usually on the same side to reduce piping and place the pumps close to the inlet and outlet. The membrane tubes include an outer cylindrical housing and a plurality of tubes having a thin membrane on the inside with the membrane tubes inserted in a closely packed manner into the housing. The membrane tubes may be positioned in the housing in a defined manner and number optionally employing a tube locator which is inserted into or on the end of each membrane tube in the module. The tubes have a potting resin injected at each tube end in the space between the closely bundled tubes and the housing as shown for example in U.S. Pat. No. 4,707,261, issued Nov. 17, 1987.
Multiple membrane tubular modules include a housing with a plurality of membrane tubes, the space between each individual membrane tube at each end potted with a potting resin to the end of the membrane tube with one open end of each individual membrane tube discharging a concentrate stream from the module and the other open end of the tubes receiving the concentrate stream from a previous module as a feed stream. The ends of the membrane tubes in each module have short end fittings which collect the flow from the individual membrane tubes and direct the flow into one end of the U-bend tube. The other end of the U-bend tube also has an end fitting which then directs the concentrate stream flow from the previous module into the individual membrane tubes of the next module as a feed stream.
Typically, the end fittings fit and are sealed in a fluid tight manner within the end of the module housing while the other end of the fitting is threaded with an outer nut which retains one end of a U-bend against a sealing washer or "O" ring within the end fitting for a fluid tight connection to one end of the U-bend. The other end of the U-bend is connected similarly in a fluid tight, threaded sealed connection with another end fitting. A trumpet-type or conical opening of the one end of the end fitting is positioned directly against and adjacent the resin potted ends of the individual tubes to receive the fluid flow from or direct the fluid flow to the individual membrane tubes in the module.
In such prior art tubular membrane modules, there is a pressure drop in the fluid between the end of the tubes which discharges the concentrate into the end fitting and connecting U-bend and into the end of the next tubes which receive the concentrate as the feed stream. Between the respective ends of the membrane tubes is a cross sectional area change and an irretrievable loss of fluid total energy due to changes in kinetic energy. Membrane module systems employ at present a trumpet-shaped or conical end fitting adjacent the ends of the membrane tubes of the membrane modules to collect the flow of one stream from the tube ends and to direct the stream into the next succeeding membrane module through the connecting U-bend. The end fitting has the wide flared trumpet end of the fitting positioned toward and in position to collect the total flow of the concentrate streams from the tube ends with the reverse for the introduction of the concentrate stream as the feed stream. The fluid discharged into the open trumpet end of the tube accelerates in flow and causes an entrance pressure loss effect, i.e. a contraction loss. When the fluid is discharged from the trumpet end of the tube and into the feed inlet, there is an expansion pressure loss effect, i.e. expansion loss. The problems associated with contraction-expansion losses in membrane modules are well recongnized and known.
It is desirable to increase the number of membrane modules that may be employed in any membrane system and to improve the separation efficiency of the system. It is also desirable to reduce the hydrodynamic pressure drop associated with each membrane module and to increase crossflow velocity since reduced pressure drop improves energy efficiency and increased flow velocity results in increased filtration rates.